Knowing the orientation of an arrangement is important in many applications, for instance in surveying when measuring the distance to a target point via a reflector pole possibly not standing vertically above the target point, in aeronautics and astronautics when docking aircraft for refueling, for instance, or in the automatic control of machines monitoring the course of a newly built road. In these areas, the position of a target point in space often is determined by optical measuring instruments such as theodolites, cameras, or laser range finders. For the measurements, often a tactile tool having light-emitting and/or light-reflecting points is positioned at the target point.
Thus, in WO 91/16598 a system for determining the condition of a surface is disclosed where the system is composed of at least two opto-electronic sensors and a tool, said tool being equipped with at least two light sources and three contact points or a contact area. The tool is fixed to a surface by means of the contact points, and the orientation of the surface is determined from the measured space coordinates of the light sources and their known positions relative to the contact points. However, for such a tool for determining the orientation, a power supply is needed at the target position, which often cannot be furnished when the targets are of difficult access.
In EP 0 880 674 a method is disclosed that can be used to determine the space coordinates of target points via a combination of laser distance measurement and photogrammetry. In this method a tactile tool having several reflecting points is positioned at a target point. A laser area finder will measure the distance to one or several reflecting points, while a camera takes a picture of the reflecting points. On top of the camera a flash lamp is fixed as a target illumination for illuminating the reflecting points. From the image of the reflecting points in the camera sensor, the position and orientation of the tactile tool are calculated. The system is optimized primarily for measurements over short distances, particularly so for the automobile and aircraft industry, since a flash lamp having a short illuminating range is used for illumination, so that the measuring range is short as well. In addition, in measurements over large distances the distance of separation becomes hard or impossible to measure, since several reflecting points will enter the field of vision of the EDM. The method also is subject to errors when using reflectors as the measuring points of the camera, rather than measuring points actively emitting radiation, since there is no coupling of the target illumination into the camera's optical axis. Here the error committed in determining the position of the measuring points is of the same order of magnitude as the distance of the target illumination source from the camera's optical axis. The method lacking actively light emitting measuring points thus will not be suitable for geodesically relevant measuring accuracies.